The invention relates to a pneumatic device for retaining and positioning a thread chain and, more particularly, to an improved vacuum latchtack throat plate with a vacuum generating apparatus.
Generally, a sewing machine for forming an overedge chain stitch has a sewing needle, loopers, and a stitch tongue for forming the thread chain, and a presser foot, feed dogs and a throat plate for cyclically advancing a work piece through the sewing needle. The stitch tongue, which acts like a knitting needle around which the thread chain is formed, is typically part of the throat plate.
In sewing a series of work pieces with a manual, semi-automatic or automatic sewing machine that produces an overedge chain stitch in the work pieces, the sewing machine continues its sewing operation for a time after the work piece has passed through a stitch forming region of the sewing machine. This continued operation of the sewing machine creates a thread chain extending back toward the sewing needles from the trailing edge of each work piece. Typically, the thread chain is severed from the work piece after the trailing edge of the work piece moves beyond the throat plate of the sewing machine.
After the thread chain is severed from the work piece, a portion of the thread chain remains attached to the sewing needle. This portion of the thread chain will then become attached to the leading edge of the next work piece passing through the sewing needle.
In order to improve the appearance of the finished work pieces and to prevent unraveling of the seam in the work pieces, the portion of the thread chain that is attached to the sewing needle must be positioned and retained upstream in the line of feed from the sewing needle. The portion of the thread chain is positioned so that it may then be oversewn into the seam an the leading edge of the next work piece. The process of oversewing the thread chain into a seam at a leading edge of a work piece is known as latchtacking.
Devices are known for retaining and positioning a portion of a thread chain to be oversewn into a seam at the leading edge of a work piece. For example, U.S. Pat. No. 4,038,933 to Marforio includes a vacuum operated device for latchtacking a thread chain. A throat plate includes a longitudinal channel, which extends through the entire length of the throat plate and a stitch tongue. The longitudinal channel is coupled by a conduit to a switching valve. The switching valve is coupled by a second conduit to a vacuum unit. A trim knife is located adjacent to the throat plate.
A disadvantage of the Marforio device is that the vacuum is supplied to the throat plate through the switching valve. The switching valve will typically have small vacuum passageways that may become clogged or partially blocked by lint, dust, thread pieces and the like, which are ingested through the stitch tongue. In addition, the location of the switching valve may increase the distance between the vacuum unit and the throat plate, thereby increasing the time required to obtain vacuum at the stitch tongue once the valve is switched. It is desirable to mount the vacuum source close to the stitch tongue and to precisely control the timing of the application of the vacuum.
A further disadvantage of the Marforio device is that the trim knife is positioned to the right of the straight longitudinal channel. The trim knife trims the edges of the work piece before the work piece reaches the sewing needles. Because the trim knife is positioned to the right of the longitudinal channel, the resulting seam width size may be unacceptable on many work pieces.
U.S. Pat. No. 5,159,889 to Price et al. also includes a vacuum operated device for latchtacking a thread chain. A throat plate includes an air conduit that terminates in an opening at a chaining tongue. The air conduit extends from the throat plate to a vacuum canister, where the air conduit is coupled to an internal control valve assembly suspended from the lid of the canister. When the internal control valve is open, reduced air pressure within the canister draws a stream of air through the air conduit. The reduced air pressure within the canister is established by coupling the canister to an inlet conduit of a venturi. The venturi is coupled to a high pressure air line by a valve.
A disadvantage of the Price device is that the vacuum canister is physically large making mounting close to the throat plate difficult. In addition, the vacuum canister requires regular maintenance to remove the lint, dust, thread pieces and the like that are drawn through the opening in the air conduit and into the canister. Furthermore, because vacuum is drawn through the internal control valve assembly, the presence of such debris on the valve element of the internal control valve assembly may prevent the internal control valve from completely closing. It is desirable to mount the vacuum source close to the stitch tongue and to minimize required maintenance.
A further disadvantage of the Price device is that two valves are utilized, the internal control valve assembly and the valve coupling the venturi to the high pressure line. It is desirable to minimize the number of components used in the vacuum system. In addition, the time required to evacuate the canister with this valve arrangement may limit the number of work pieces that can be produced in a given time.
Another known device for retaining and positioning a portion of a thread chain to be oversewn into a seam at the leading edge of a work piece is available from Atlanta Attachment Company of Lawrenceville, Ga. The device includes a thin-walled vacuum tube that is soldered to the underside of the throat plate and stitch tongue. The thin-walled vacuum tube is soldered to a vacuum conduit at the front edge of the throat plate.
A disadvantage of this device is that the stitch tongue may not be hardened and polished after the thin-walled vacuum tubing is soldered in place. It is desirable to harden and polish all surfaces of the stitch tongue because the thread chain is formed around the stitch tongue and should smoothly slide over the stitch tongue. Surfaces of the stitch tongue that have not been hardened are subject to needle nicks, which may cause the thread chain to snag and necessitate replacement of the entire throat plate. In addition, with the thin-walled tubing, which is formed and then soldered in place, the stitch tongues on a series of throat plates may not be consistently dimensioned, leading to greater set up times when replacing a throat plate. Furthermore, the vacuum conduit may not be readily reformed or repositioned.
Accordingly, it would be desirable to have an improved vacuum latchtack throat plate and vacuum generating apparatus.